1. Field of the Invention
The present invention relates to electrical fuses for high voltage underground distribution systems, and more particularly relates to a novel encapsulated fuse assembly which provides an effective corona shield.
2. Description of the Prior Art
It is desirable to provide underground power distribution system components with fuse protection to prevent damage to such components when current surges occur on the system. However, it has been found that underground power distribution systems produce severe corona problems when fuses are used in such systems.
Referring to FIG. 1, a conventional fuse assembly 100 is provided with an insulative housing 101 and an electrically conductive ground shield 102 on its outer surface which is in contact with the earth in which it is buried. As a result, steep voltage gradients across the insulating material of the fuse assembly are formed. The high system voltages present in the fuse 103 are separated from the ground shield 102 by a relatively thin insulating material. Under these conditions there is a tendency for the fuse to become electrically stressed and corona to discharge or arc from the fuse elements. This results in the formation of nitrous oxides which attack the metal components of the fuse. After the fuse has been subjected to such action for a long period of time, it may become severely corroded and the proper operation of the fuse under short circuit conditions may be seriously impaired.
Accordingly, it is desirable to provide fuses with a corona shield to reduce electrical stress and prevent arcing. Such shields operate to distribute the electrical stress across the shield and around the fuse. Thus, voltage gradients along the fuse elements are reduced and arcing is prevented. Naturally, it is important that the shield not provide an alternate electrical path between the fuse terminals when the fuse is open.
Devices to prevent corona discharge from a fuse are known. For example, U.S. Pat. No. 3,946,351 to Bronikowski et al. discloses a shielded fuse assembly comprising two housing halves which are joined to encapsulate an electrical fuse. A corona shield is imbedded within each housing half and is in electrical contact with a terminal of the fuse. A gasket is provided between the halves to prevent electrical contact between the shields.
Similarly, U.S. Pat. No. 3,818,407 to Edgerton discloses a fuse enclosure including first and second conductive shield members. Each shield member extends longitudinally from one terminal of the enclosed fuse toward the other. The conductive shield members envelop the fuse and overlap each other but do not make contact. A similar conventional shielding arrangement is shown in FIG. 1. Disposed within the insulative housing 101 are separately molded conductive members 104 and 105, each being in electrical contact with an adjacent fuse terminal 106. The conductive members 104 and 105 surround the fuse 103 but are arranged within the housing 101 such that the conductive members do not contact each other. The resulting gap 107 prevents the flow of current between the fuse terminals through the conductive members.
These and other conventional shielding arrangements involve adding one or more separate shielding components to the fuse assembly. This results in relatively high manufacturing costs and an increase in the overall size of the fuse assembly. Accordingly, there is a need for an encapsulated fuse assembly having a corona shield which is relatively inexpensive to manufacture and at the same time conveniently compact to allow its use in the relatively confined passageways available for mounting such housings in underground power distribution systems.